1. Field of the Invention
This invention relates to improved methods and apparatus for feeding a catalyst into a polymerization autoclave. The subject invention finds particular utility and offers outstanding advantages in conducting continuous high pressure processes for polymerizing ethylene in the presence of free-radical generating catalysts.
2. Description of the Prior Art
Because of the powerful influence exerted by the catalysts used therein, the successful operation of polymerization processes is often critically dependent upon the accuracy and reproducibility of methods used in handling and feeding such catalysts and the maintenance of correct and controllable levels of catalyst activity during the polymerization process. The practical problems encountered in attaining such uniform and reproducible results are usually accentuated by the small concentrations at which catalysts are normally used and the consequent low feed rates at which they are supplied to the polymerization process. For example, in the widely commercialized high pressure method of polymerizing ethylene, the preferred free-radical generating catalysts are seldom employed in concentrations of over 1000 parts per million parts of total monomer by weight, and no more than 100 parts per million are usually needed when polymerization temperatures reach the conventional steady state levels, exceeding 100.degree. C.
U.S. Pat. No. 2,897,183 to Christl and Roedel outlines the more prominent techniques which have been used commercially for high pressure polymerization of ethylene. Said patent also adequately describes representative conditions for operating the so-called continuous, stirred autoclave technique for the high pressure polymerization of ethylene. As indicated in U.S. Pat. No. 2,897,183, such a well-mixed autoclave reactor affords about the closest practical approach to a constant environment high pressure process for polymerizing ethylene. Thus, in this technique, once start-up procedures are over and "light-off" (i.e. active, free-radical induced polymerization rate) has been attained, a substantially steady state, continuous process is desirably maintained with nearly constant operating temperatures and pressures, and steady monomer feed rates and polymer production rates.
Reaction conditions and flow rates are usually regulated to accomplish from about 5 to about 25% conversion of monomer to polymer per pass through the stirred autoclave, involving average residence times therein which can run from less than 1 minute to several minutes. After the product stream leaves the autoclave, it is continually processed to remove solid polymer product and the unreacted monomer is recycled back to the polymerization autoclave, undergoing suitable purification, drying, recompression and/or cooling treatments in route.
Accordingly, it will be seen that once "light-off" of the polymerization reaction has occurred the maintenance of the steady state operation desired in this continuous, stirred autoclave process will depend largely upon striking the proper heat balance between the exothermic heat of polymerization of the ethylene fraction reacting and the heat capacity of the reaction mass, a most significant contribution to which is made by the fraction of ethylene which is merely heated up from its inlet temperature while passing through unreacted. In practice the usual approach to regulating this extent of polymerization and striking this heat balance has been through modification of the catalyst feed stream to the process. For example, automatic feed back control based upon temperatures sensors located in the polymerization autoclave is often used for this purpose. However, the high operating pressures and the minute amounts of catalyst involved increase the dangers of encountering flow irregularities and instabilities which might interrupt, undermine or interfere with the intended control system.
In spite of these inherent problems in handling and feeding catalyst streams, most continuous, stirred autoclave polymerizations appear to have been conducted heretofore by merely injecting the catalyst stream or streams directly into the autoclave, e.g. as shown in U.S. Pat. No. 2,728,753 and U.S. Pat. No. 2,964,515. On the other hand, some prior art patents such as U.S. Pat. No. 2,475,643 and U.S. Pat. No. 2,897,183 (discussed above) have suggested introducing the catalyst stream into a monomer stream approaching the autoclave so as to premix the two components before they actually enter the polymerization zone of the autoclave. Although this latter approach tends to improve the distribution of the catalyst and minimize the occurence of hot spots in the polymerization zone, it increases the danger of premature polymerization and consequent fouling of the feed lines with deposits of solid polymer and also tends to reduce the sensitivity and responsiveness of any automatic feed back control systems based upon temperature sensors located in the polymerization zone.